In recent years, a low-emission vehicle has been known to which a gas fuel tank filled with high pressure gas fuel is mounted. For example, Japanese Patent Laid-Open No. 2000-127860 discloses a bus to which a CNG cylinder filled with compression natural gas (CNG) as gas fuel is mounted. This bus is such that a roof cover that covers a roof forms a cylinder housing space that houses the CNG cylinder, and a gap is provided between a lower edge of a front surface of the roof cover and the roof. When the bus runs, air flows into the cylinder housing space through the gap, and the air having passed through the cylinder housing space flows outside from an air outflow portion provided in a rear surface of the roof cover.